Add an "unsupported" surface type to SphericalCoordinates

[chapulina]

Desired behavior

During gazebosim/gz-msgs#177 (comment), it was brought up that it would be convenient to provide an unsupported type to the SurfaceType enum so that we can explicitly express that an invalid type has been passed.

Alternatives considered

Keep it the way it is and force the class to always be valid. This makes it difficult to detect configuration errors.

Implementation suggestion

Add an enum at the end and keep it at the end as we add new types.

Additional context

This is something to consider not only for this enum, but for all our enums too.